The present invention relates generally to wireless networks and, more particularly, to interconnecting wireless field device networks that are separated either logically or physically from each other by a barrier to wireless communication.
A wireless field device network includes of a cloud of devices or nodes with a central controller or gateway. The nodes in the wireless network are able to both send and receive information. In a star network, exemplified by the popular Bluetooth® format, the reach of the network is limited by the transmission range of a master device. All communications from slave devices are routed through the master according to the master's communications schedule. The range of the wireless star network can be extended by allowing the slave devices to be members of different networks so that communications can be relayed from master to slave device through multiple networks in a scatternet fashion. Star networks can also create a genealogical relationship among interconnected networks; slave devices becoming master devices of child networks. The use of a star network topology imposes some inefficiency in the routing of communications as there is rigidity to the master-slave relationship that can force messages to take a sub-optimal path to the destination node.
Mesh networking is a more flexible network architecture that is becoming prevalent in industrial applications. A mesh network includes a cloud of nodes and a controller or gateway, but avoids many of the limitations of the star network topology by allowing neighboring nodes within the same network to communicate directly with each other, avoiding unnecessary routing of communications to the controller. Each node is assigned multiple communications pathways that are interchanged to compensate for bottlenecks and linkage failures. By allowing neighboring nodes to form communications relays directly to the target node, and by routing around failures or bottlenecks, network response time is improved while minimizing network power usage by minimizing the number of transmissions required to relay a message. Utilizing multiple communications pathways provides path diversity which improves network reliability. Mesh networks can also communicate with each other by sharing nodes. These shared nodes can keep the communications schedule of every network in which it is a part, using an algorithm to determine priority between networks when there is a conflict.
Wireless networks are independent when the wireless networks do not or cannot communicate with each other. A barrier to communication can range from physical obstacles, such as long distances to natural obstructions (such as hills or trees), or man made impediments (such as concrete construction), or to logical problems inherent to the networks, such as differences in network protocols. The lack of communication between networks is inefficient, and potentially dangerous. When control systems are unable to monitor subsystems located in different independent networks, the utility of a centrally located control system is dramatically reduced. The challenge of linking disparate independent networks is that the solution must be flexible, reliable, and effective while being inexpensive. Common methods of interconnecting independent wireless networks, such as using gateways connected by an intranet backbone or by a separate wireless backbone (e.g., Wi-Fi), or radio repeaters, or home run network cabling require large outlays for installation of expensive wiring and equipment. Another issue with these mechanisms is that they often require an external power source, not readily available or practicable in many areas that are serviced by wireless field device networks.
The term “field device” refers to any field-mounted device that performs a function in a control or process monitoring system or plant monitoring system, including all devices used in the measurement, control and monitoring of industrial plants, processes or process equipment, including plant environmental, health and safety devices. A field device typically includes a sensor or an actuator or both and may perform a control or alert function. In wireless network systems designed for sensor/actuator-based applications, many devices in the network must be locally-powered because power utilities, such as 120V AC utilities or powered data buses, are not located nearby or are not allowed into hazardous locations where instrumentation, sensors, and actuators and safety monitors or human interface devices must be located without incurring great installation expense. “Locally-powered” means powered by a local power source, such as a self-contained electrochemical source (e.g., long-life batteries or fuel cells) or by a low-power energy-scavenging power source (e.g., vibration, solar, or thermoelectric). A common characteristic of local power sources is their limited energy capacity or limited power capacity, either stored, as in the case of a long-life battery, or produced, as in the case of a solar panel. Often, the economic need for low installation cost drives the need for battery-powered devices communicating as part of a wireless field device network. Effective utilization of a limited power source, such as a primary cell battery which cannot be recharged, is vital for a well functioning wireless field device. Batteries are expected to last more than five years and preferably last as long as the life of the product.
In order to save power, some wireless network protocols limit the amount of traffic any node or device can handle during any period of time by only turning their transceivers ON for limited amounts of time to listen for messages. Thus, to reduce average power, the protocol may allow duty-cycling of the transceivers between ON and OFF states. Some wireless network protocols may use a global duty cycle to save power such that the entire network is ON and OFF at the same time. Other protocols (e.g., TDMA-based protocols) may use a local duty cycle where only the communicating pair of nodes that are linked together are scheduled to turn ON and OFF in a synchronized fashion at predetermined times. Typically, the link is predetermined by assigning the pair of nodes a specific time slot for communications, an RF frequency channel to be used by the transceivers, who is to be receiving, and who is to be transmitting at that moment in time (e.g., a TDMA with channel hopping protocol, such as WirelessHART®).
Wireless field device networks are used to control and monitor disparate processes and environments. For example, wireless field device networks may be used in oil fields. An oil field is composed of numerous discrete locations centered on well pads that are scattered over large areas. Communication between these isolated local areas is essential to the overall management of the field. The wireless field device network at a well pad monitors and controls everything from flow rates and fluid temperature to valve status and position and potential leaks. The resulting data is relayed through the network to controllers that analyze the data and actuate control mechanisms in order to manage production or prevent trouble. Home run cabling from each isolated well pad to a centrally monitored station may be impractically expensive, so often times a wireless Supervisory and Control Data Acquisition system (SCADA) is employed to connect the well pads together into a star network. However, SCADA systems are expensive to install and often require expensive solar panels with battery back-up to power them. The oil field environment can be extremely difficult for a wireless mesh network to operate reliably. Distances between well pads are often greater than the standard range of a wireless field device and there are often physical obstructions to wireless communication, such as earthen berms, tanks, processing equipment, rocker/rod pumps and sheds. Wireless links between networks are often blocked by natural vegetation, such as trees and bushes. Trees absorb 2.4 GHz spectrum radio emissions at ˜0.35 dB/m, rapidly consuming the link budget of low powered radios used in wireless networks. RF signal intensity is often insufficient with low level and low powered wireless field devices to cover the distances between well pads and overcome the obstacles required to communicate with other wireless field device networks. These well pad locations are typically remote, limiting ready access to convenient sources of electricity. This limits the power of the wireless transceiver that can be installed: without an external supply of electricity, a more powerful transceiver will drain stored energy rapidly and quickly run up replacement and maintenance costs while causing frequent interruptions of network interconnectivity.
One method for interconnecting scattered independent networks is to install a gateway or base station device within each network and link each network through a hardwired backbone or through a separate wireless backbone (e.g., Wi-Fi or a proprietary point-to-point RF network). There are two primary disadvantages to such devices: they are expensive and they are energy intensive devices. Remote locations may not have sufficient power sources available to operate the gateway or base station for long periods without frequent maintenance. There are also additional costs associated with the gateway or base station, which are typically expensive, and require large power sources. Placing cable runs throughout a network scattered over large areas is difficult and can be prohibitively expensive.
To avoid the difficulties of installing a separate communications backbone, network systems may use wireless repeaters to increase range or overcome obstructions. A repeater is a high powered device that functions by transmitting a received low powered signal at much higher power so that the signal will overcome obstructions and distant networks can detect it. Such repeaters require external power sources in order to provide the necessary amplification. Remote locations may not have sufficient power sources available to operate the repeater.
In applications where physical space constraints are a factor, one solution to overcoming an obstruction between two networks is to use a radio transceiver with two antennas. By placing a separate antenna in each area to be interconnected, a single radio transceiver can send and receive signals in both areas. External RF cables connect each of the two antennas to the radio through a powered or passive RF splitter. The use of external RF cables and an RF splitter imposes severe restraints on the length of cable used. For example, LMR-400 low loss RF cable will incur a signal loss of ˜0.22 dB/meter at 2.4 GHz and a passive RF splitter reduces the available RF signal strength at each output by half. Practically, this solution can only interconnect two networks, and these networks must be very close to each other before signal loss renders the device useless. Substituting an RF switch for the RF splitter and switching the RF signal between the two antenna's recovers some of the lost signal but adds considerable complexity to the wireless device and still leaves large losses associated with the extra RF antenna cable.